At present, the penetration of intermittent renewables in most power grids is low, but wind for example has reached about 17.5% in Denmark in a normal wind year (where plans are underway to increase this to in excess of 50% by 2025 ) and 7% in Germany. In small amounts, integration of intermittent power sources has little effect on grid operations, but the more variable and intermittent nature of power generation from these renewable sources compared to conventional sources has raised concerns, by no means universal, about the ability of electricity grids to absorb large amounts of this intermittent power to replace the output of conventional power sources, and the economic implications. However technological solutions to deal with intermittency already exist see Control of the National Grid (UK), and studies by academics and grid operators indicate that the cost of compensating for intermittency is expected to be low even at levels of penetration substantially higher than those prevailing today. For example, in evidence to the House of Lords Economic Affairs Select Committee, the UK System Operator, National Grid have quoted estimates of balancing costs for 40% wind and these lie in the range £500-1000M per annum. "These balancing costs represent an additional £6 to £12 per annum on average consumer electricity bill of around £390

Given that power demand on existing grids varies substantially and on different time scales (seasonally, daily, for weather-related reasons and the like), matching power demand to supply is not a problem specific to intermittent power sources. All generating sources have some element of randomness (including unforeseen breakdowns) and differing capabilities to vary output or shed loads in response to either sudden and large changes demand or sudden and large changes in available power generation. Hence power grids are already designed to have some capacity in excess of projected peak demand, including for sudden loss of conventional power plants.

As a report from the IEA puts it
"In the case of wind power, operational reserve is the additional generating reserve needed to ensure that differences between forecast and actual volumes of generation and demand can be met. Again, it has to be noted that already significant amounts of this reserve are operating on the grid due to the general safety and quality demands of the grid. Wind imposes additional demands only inasmuch as it increases variability and unpredictability. However, these factors are nothing completely new to system operators. By adding another variable, wind power changes the degree of uncertainty, but not the kind - a fact that several authors referred to recently (DeMeo et al. (2003))."

In the context of global warming, some argue that the main benefit of renewable energy sources is reducing overall emissions by displacing fossil fuel generation. Others argue that most power generation, particularly electricity, can be generated from renewable (intermittent) sources using existing technology.

A series of detailed modelling studies by Dr. Gregor Czisch, which looked at the European wide adoption of renewable energy and interlinking power grids using HVDC cables, indicates that the entire European power usage could come from renewables, with 70% total energy from wind at the same sort of costs or lower than at present. Intermittency can clearly be dealt with, according to this model by a combination of geographic dispersion to de link weather system effects, and the ability of HVDC to shift power from windy areas to non-windy areas.

Terminology

Several key terms are useful for understanding the issue of significantly intermittent power sources. These terms are not standardized, and variations may be used. Most of these terms also apply to traditional generating plant.

Nominal or nameplate capacity: This is the most common number used for referring to the normal maximum output of a generating source. For example, a wind turbine may be referred to as a 1.5 MW turbine, or a 120 MW wind farm.

Capacity factor or average capacity factor: the average expected output of a generator, usually over an annual period. Generally stated as a percentage of the nameplate capacity. For wind, capacity factors are generally between 25-40% of the nameplate capacity, depending on the local wind resource.

Capacity credit: generally, the amount of output that may be statistically relied upon during periods of peak demand. This figure will depend on the correlation of local production and demand features, and will not be directly comparable between different grids. Crucially, the correlation between different generating facilities is a key factor in determining overall system reliability. If different wind farms, for example, can separately be relied upon to produce 10% of peak demand, but they have low correlations between them because they are located far apart, their combined capacity contribution will be higher than 10% due to diversification. Estimates of the capacity credit for wind vary from 10-30%, although 10% is used in some locations as a baseline figure (due to lack of sufficient historical data, for example).

Penetration or peak penetration refers to the nominal capacity over estimated peak demand in the system grid (for example, of wind-generated power) in percentage. As noted below, this figure does not provide much information beyond the scale of the amount of (potential) wind generation to peak demand. Most large systems have wind penetration of substantially less than 5%, although Denmark has approximately 44%, Spain 26%, Portugal 24%, and Germany 23%. At much higher penetrations, more appropriate measures and corrections to the gross penetration have been proposed to better characterize the issue of intermittency. Penetration may also be used to refer to the amount of energy generated as a percentage of annual consumption.

Maneuverability or dispatchability: The ability of a given power source to increase and/or decrease output quickly on demand. The concept is distinct from intermittency; maneuverability allows grid operators to match output (supply) to system demand. For example, nuclear power is generally run at close to full output over long periods (months), and other generators must compensate. Gas-fired stations can generally increase or decrease output quickly, and coal-fired stations can increase or decrease output, but less quickly than gas (note that certain types of thermal generators, particularly CHP, produce heat that may be needed for other uses and hence be less maneuverable, except of course if as is usually the case in Scandinavia, where heat storage and heat only boilers are used). Hydroelectricity is typically highly maneuverable, as long as water is available. Wind power both is intermittent (variable) and non-maneuverable, unless combined with some form of commercial-scale energy storage such as pumped-storage hydroelectricity.

Intermittency of various power sources

General Problems

Intermittent losses from power sources can be caused by a number of events:

Unplanned outages (accounted for 1.6% of worldwide nuclear power plant capacity in 2006)

Component failures

Design flaws

Grid disconnects

Natural disasters

Terrorism

All of these can cause an immediate loss of power production and is true intermittency, a particular type of variability that switches between full power and no power (however it is hard to see how this true intermittency can apply to the main contenders for mass renewable replacements of fossil plant such as wind and solar, since the wind and sun cannot suddenly disappear simultaneously for a large numbers of necessarily small renewable sources). Several conventional units can fail without warning (a recent case involved the sudden but unconnected failure of Sizewell B nuclear power station and Longannet coal fired station losing 1.5 GW and causing power failures ) and simultaneously if a common failure mechanism arises such as an earthquake, an airburst nuclear explosion, a grid disconnect, or a common design flaw manifests or is discovered.

Transmission lines may also break down unexpectedly, the result of over loading, or a tree falling, or bushfires, lightning strike, icing, high winds, providing a source of intermittency in existing grids by the disconnection of major power stations. Wind blown debris caused wide spread black outs in the UK storms of 1987.

Solar energy

Intermittency inherently affects solar energy, as the production of electricity from solar sources depends on the amount of light energy in a given location. At current penetration levels, solar energy presents few issues for integration into existing electricity grids, and discussion of potential issues is highly theoretical.

The extent to which the intermittency of solar-generated electricity is an issue will depend to some extent on the degree to which the generation profile of solar corresponds to demand cycles. In areas with high solar production possibilities, and where air conditioning is a driver of demand and corresponds to periods of high sunlight, variability may actually be beneficial. For example, solar thermal power plants designed for solar-only generation (such as Nevada Solar One) are ideally matched to summer noon peak loads in prosperous areas with significant cooling demands, such as the south-western United States. Using thermal energy storage systems, solar thermal operating periods can even be extended to meet base-load needs.

Using a variety of energy sources in combination can help to overcome intermittency. For example, stormy weather, bad for direct solar collection, is generally good for wind power.

Wind energy

Wind-generated power is a variable resource, and the amount of wind-generated electricity produced at any given point in time by a given plant will depend on wind speeds and turbine characteristics (among other factors). While the output from a single turbine can vary greatly and rapidly as local wind speeds vary, as more turbines are connected over larger and larger areas, the slower and less variable the aggregate rate of change becomes. A study of wind in the United States demonstrated that any ten wind farms connected through the grid could be relied upon for from 33 to 47% of their average output as reliable, baseload power.

A quite different point, is that as the number of individual wind units need to replace a single large conventional unit is very high, these cannot all fail mechanical ally or otherwise simultaneously meaning that in fact a large number of wind turbines is more reliable than the equivalent conventional plant. For example parts of UK were blacked out recently due to the near simultaneous failure of two large power plants, losing 2.5 GW in 10 minutes. This simply could not occur due to simultaneous failure of the 2,500 x 3 M wind turbines (on average) that would replace this output.

As compared to many other types of electricity generation, wind is not normally dispatchable - it cannot be turned on at will by human or automatic dispatch to meet increased demand. Variability may be a more accurate term to describe wind's generation profile than intermittency, which implies an alternating presence or absence (generation that is either on or off). In discussions of the pros and cons of wind power, the issue of variable power output may be termed intermittency or variability without distinction between the two terms.

Much is made of the performance of wind power during heat waves by some commentators, because that is typically the yearly peak electricity demand for most temperate to hot climates. A study during the 2006 California heat storm revealed that output from wind power in California significantly decreased to less than 5% during peak demand. A similar result was seen during the 2003 European heat wave, when the output of wind power in Germany fell to 10% during peak demand times, resulting in importing a peak of around 2,000 MW of electricity. However it is well recognised that this kind of situation can be dealt with by adequate planning of reserves.

According to an article in EnergyPulse, "the development and expansion of well-functioning day-ahead and real time markets will provide an effective means of dealing with the variability of wind generation." However, the experience of Denmark, which has one of the greatest percentages of wind power utilisation in the world, would suggest otherwise. The ICE report on the Danish experience stated that wind power was so variable that Denmark exported most of its wind power, rather than use it itself. In addition, in 2002 the entire system had a total of 54 days without usable power generation. The report concluded that it would be difficult for "island" grid countries like Britain to use a large percentage of wind power in combination with slower reacting thermal power stations. A similar report by the Renewable Energy Foundation confirms the problems experienced in Denmark and goes on to indicate that the UK may experience a rise in CO2 emissions through using wind power:

Denmark achieves little or no direct reduction of emissions, because its CO2-free wind power is working alongside CO2-free hydro-power ... operating fossil capacity in (standby) mode generates more CO2 per kWh generated than if operating normally.

In the UK serous academic commentators such as Graham Sinden, of Oxford University argue that this issue of capacity credit is a "red herring" in that the value of wind generation is largely due to the value of displaced fuel, not any perceived capacity credit - it being well understood by the wind energy proponents that conventional capacity will be retained to "fill in" during periods of low or no wind.

Critics also argue that the economics of wind energy may be challenged when wind production is high at times of low demand. Due to the presence of other generating stations that are operated as base load (run as close to continuously as possible) or have minimum operating cycles, at high penetrations wind plants may contribute to the grid producing energy "surplus" to immediate local demand. As with other generating plant, wind energy output can be stored for future use, or may on occasion need to be curtailed or demand increased to compensate. While all of these solutions are commonly used to manage grids, wind "spilt" or curtailed generates no revenue, and prices for supply to the grid may be lower at times of high output, both of which may make both wind farms and dispatchable power plants less profitable. Energy storage used to arbitrage between periods of low and high demand always incurs some efficiency losses, though helps to mitigate demand peaks.

Proponents argue that since a conventional dispatchable plant can be and is routinely cycled, operators may curtail its output rather than a wind plant. All conventional power plants have limits to the extent to which they can be cycled up and down and over different time periods and efficiency limits in certain circumstances. Although import and export capacity may be limited, surplus power may also be sold to neighbouring grids and re-imported at times of shortfall.

Both shortfalls and surpluses of supply attributable to wind energy's variability will be less frequent at lower penetration levels. At low to medium levels of penetration (up to 15%), incremental regulation and operational reserve requirements are generally marginal, and demands for reduced supply (curtailment) infrequent. At lower levels (less than 5%), wind may simply be treated as "negative load" in the larger system or statistical "noise" in a large system. Very few grids have wind energy penetration above these levels. Many studies have considered penetration above these levels: a Minnesota study considered penetration of up to 25%, and concluded that integration issues would be manageable and have incremental costs of less than one-half cent ($0.0045) per kWh.

A diversity of renewable energy sources, each serving fewer and nearer users, would also greatly restrict the area blacked out if a grid connecting them failed. And when renewable energy sources do fail, they generally fail for shorter periods than do large power plants.

Hydroelectricity

Hydroelectric power is usually extremely dispatchable and more reliable than other renewable energy sources. Many dams can provide hundreds of megawatts within seconds of demand (see Ffestiniog power station). The exact nature of the power availability depends on the type of plant.

In conventional hydroelectric plants, there is a reservoir and a one-way generator. The water level of the reservoir can be adjusted frequently to meet changing demand throughout the day by running the generator when demand is high and not running it when demand is low.

Pumped-storage hydroelectricity can make an even more significant contribution to peaking ability of the grid. These just move water between reservoirs and are powered by power from the grid when demand is low and put power back into the grid when demand is high. There also exist combined pump-storage plants that use natural flow from some river as well as extra pumping when demand is low.

Direct pumped-storage doesn't even contribute any net generation to the grid, in fact, it increases the fuel used by other power plants because there is inefficiency in the turbine/generator. The economic benefit of pumped-storage plants lies only in increasing the capacity of the grid. This type of plant works well on a grid with many nuclear or renewable energy plants because the fuel is very cheap or essentially free, so it costs very little to keep them running at high power during the night when demand is low. Both pump-storage plants and natural flow hydro plants can help allow for intermittency of other plants by running at higher capacity for short times, but assistance is limited by the total capacity of the hydroelectric plant.

However, it was stated in New Scientist,June 2008 that China has built 19 GW of hydro power in the Sichuan province earthquake zone, and that in principle, this could all be lost simultaneously by a catastrophic earthquake.

Conventional power stations

Once a conventional power station has come offline it may stay that way for more than a week.

Conventional power plants (as well as nuclear plants) use water for cooling, and water shortages during hot summer months has occasionally resulted in periods when output has had to be curtailed, notably in France in 2006.

Gas-fired generation

Gas-fired plants are typically very reliable and dispatchable. These kinds of plants also often have the ability to quickly vary their output to adjust to the frequent jumps and changes in consumer demand. Thus these are very good as peaking units. These benefits are weighted against the high price of gas when deployed in the grid.

Nuclear power

Nuclear power is considered a base load power source, in that its output is nearly constant and other types of plants are adjusted with changes in demand. This is done because output changes can only be made in small increments, and because of small fuel costs - there is little marginal cost between running at a low power and a high power, therefore it is cheapest for the system to run the nuclear plants at high power. Every year or two (depending on the plant), the plant must be shut down for planned outages for about a month. This is typically done in the spring or fall when electricity demand is lower, as such, on a national scale power output from nuclear increases corresponding with demand during the peak summer and winter months. This change in output commonly occurs on a yearly scale, it is rare that nuclear power plants adjust their power output to correspond with demand on a daily basis, which would be much more likely to happen in countries where over 50% of their power comes from nuclear (such as France).

Nuclear power plants are also subject to unplanned outages as noted above. For instance, unplanned outages in the United States caused a total capacity loss of 1.7% in 2000, which was drastically improved from 11.6% in 1980.

In the UK, the largest nuclear plant, Sizewell B, of total output 1.32 GW regularly suffers unplanned outages which sets the required spinning reserve margin on the total UK generating system due to it being the largest single intermittent source. The plant typically has a load factor of 80 to 85%.

In the UK, the spring of 2006 and the fall of 2007 saw times when half the nuclear capacity in the country was offline due to a combination of planned and unplanned outages. the relevance of this, is that every grid system has to allow sufficient reserves for the sudden loss of large numbers of conventional plant - and this reserve can equally well be made available to variable and intermittent renewable sources, at no extra cost, since it is already existing.

Diesel Engine Generation

It is not commonly appreciated (even amongst industry experts) that small high speed diesels (of automotive or rail traction sizes at 200kW - 1.2 MW ) as opposed to large slow speed marine diesels, have been and are very commonly used within large power grids throughout North America and Europe. France uses about 5 GW of such diesels to cover the intermittency of their nuclear stations; these are all in private hands - at small scales factories and the like - with their usage being triggered semi-randomly by a special tariff - EJP - which encourages these users to start their diesels. Large users such as CERN cut power usage as required by the System Operator - EDF.

In USA and UK these diesels have usually been purchased for other reasons e.g. for emergency standby, in water works, hotels, hospitals, etc. and in some cases for electricity substations - e.g. Cuyahoga Falls, USA (10 x 1.6 MW Caterpillar) and Tregarron Mid Wales UK (3 x 1.6 MW Caterpillar), but can be readily used to automatically synchronize and feed into the grid.

In the UK 500 MW of such plant is routinely started within a few minutes; this is perfectly acceptable to the engines' service life in a scheme operated by National Grid called National Grid (UK) reserve service . It has been established that there is 20 GW of such diesel plant in the UK and it has been pointed out that there is no technical reason why this quantity could not be brought into the Reserve Service scheme to assist handling very rapid changes in renewable output, whilst conventional plant is started or indeed stopped.

"Renewable Electricity and the Grid - the challenge of variability" Pub Earthscan. London ISBN 13:978-1-84407-418-1

The above based on a conference:

"Integrating renewables into the electricity system"
A one day conference on Tuesday January 24th at the
Open University, Milton Keynes: http://eeru.open.ac.uk/conferences.htm (viewable on line).

Power Convention 2007 10 - 11 September Imperial College, London 2007 covers these points as well.

Economic impacts of variability

Estimates of the cost of wind energy may include estimates of the "external" costs of wind variability, or be limited to the cost of production. All electrical plant has costs that are separate from the cost of production, including, for example, the cost of any necessary transmission capacity or reserve capacity in case of loss of generating capacity. Many types of generation, particularly fossil fuel derived, will also have cost externalities such as pollution, greenhouse gas emission, and habitat destruction which are generally not directly accounted for. The magnitude of the economic impacts is debated and will vary by location, but is expected to rise with higher penetration levels. At low penetration levels, costs such as operating reserve and balancing costs are believed to be insignificant.

Intermittency may introduce additional costs that are distinct from or of a different magnitude than for traditional generation types. These may include:

Transmission capacity: transmission capacity may be more expensive than for nuclear and coal generating capacity due to lower load factors. Transmission capacity will generally be sized to projected peak output, but average capacity for wind will be significantly lower, raising cost per unit of energy actually transmitted.

Additional operating reserve: if additional wind does not correspond to demand patterns, additional operating reserve may be required compared to other generating types, resulting in higher capital costs for additional plants. Contrary to statements that all wind must be backed by an equal amount of "back-up capacity", intermittent generators contribute to base capacity "as long as there is some probability of output during peak periods." Back-up capacity is not attributed to individual generators, as back-up or operating reserve "only have meaning at the system level.

Balancing costs: to maintain grid stability, some additional costs may be incurred for balancing of load with demand. The ability of the grid to balance supply with demand will depend on the rate of change of the amount of energy produced (by wind, for example) and the ability of other sources to ramp production up or scale production down. Balancing costs have generally been found to be low.

Storage, export and load management: at high penetrations (more than 30%), solutions (described below) for dealing with high output of wind during periods of low demand may be required. These may require additional capital expenditures, or result in lower marginal income for wind producers.

a detailed study for the UK National Grid states "We have estimated that for the case with 8000MW of wind needed to meet the 10% renewables target for 2010, balancing costs can be expected to increase by around £2 per MWh of wind production. This would represent an additional £40million per annum, just over 10% of existing annual balancing costs." This is a paltry £0.11/MWh.

An official at Xcel energy claimed in December 2006 that at 20 percent penetration, additional standby generators to compensate for wind would cost $8 per MWh, adding between 13% and 16% to the $50-$60 cost per MWh of wind energy. Estimates from other sources have been lower, perhaps reflecting matters specific to that company's operating conditions.

In evidence to the UK House of Lords Economic Affairs Select Committee, National Grid have quoted estimates of balancing costs for 40% wind and these lie in the range £500-1000M per annum. "These balancing costs represent an additional £6 to £12 per annum on average consumer electricity bill of around £390."

Penetration

Penetration is most frequently cited in terms of nameplate (nominal maximum) capacity of wind to peak demand, generally uncorrected for actual production. Penetration may also be referred to as a percentage of annual production (or demand), which takes into account the actual or expected output of electricity.

At high penetrations, the expected peak production of wind-generated electricity may be more important; while there is no generally accepted measure of the relevant proportion, measures may include expected peak wind output minus firm export capacity over minimum demand levels (and possibly corrected for minimum base-load generation that cannot be economically shut down, such as nuclear). All of these figures should be treated and used with caution, as the relevance or significance (or any implied limits) will be highly dependent on local factors, grid structure and management, and existing generation capacity.

There is no generally accepted maximum level of penetration, as each system's capacity to compensate for intermittency differs, and the systems themselves will change over time. For most systems worldwide, existing penetration levels are significantly lower than practical or theoretical maximums; for example, a UK study found that "it is clear that intermittent generation need not compromise electricity system reliability at any level of penetration foreseeable in Britain over the next 20 years, although it may increase costs. As of 2006, Denmark has more than 40% penetration and at least two other countries (Portugal and Germany) have penetration levels (nominal to peak demand) of more than 20%.

As the fraction of energy produced by wind ("penetration") increases, different technical and economic factors affect the need for grid energy storage facilities, demand side management, grid import/export, and/or other management of system load. Large networks, connected to multiple wind plants at widely separated geographic locations, may accept a higher penetration of wind than small networks or those without storage systems or economical methods of compensating for the variability of wind. In systems with significant amounts of existing pumped storage, hydropower or other peaking power plants, such as natural gas-fired power plants, this proportion may be higher. Isolated, relatively small systems with only a few wind plants may only be stable and economic with a lower fraction of wind energy (e.g. Ireland), although mixed wind/diesel systems have been used in isolated communities with success at relatively high penetration levels.

In jurisdictions where the price paid to producers for electricity is based on market mechanisms, compensation to producers per unit is higher when they produce when demand is high and production low. The profitability of wind farms will therefore be higher if their production schedule coincides with these periods. If wind represents a significant portion of supply and wind farm output is highly correlated, overall revenues could be lower. In economic terms, the marginal revenue of the wind sector as penetration increases may diminish.

If wind and other generating sources significantly exceed demand and mechanisms to export, store or otherwise divert this energy are insufficient, wind turbines may have to curtail their output (for example, by changing the pitch of the turbine blades). This is a normal operating procedure that can be handled by turbine operators and control software. It reduces, however, the revenue generated by the wind plant and will affect the economic viability of wind production. In some cases, grid pricing procedures may allow for nil or negative prices, providing incentives to market participants to curtail production or increase load (for example, for storage).

Although penetration is generally stated in terms of nameplate capacity (peak output) over peak demand, at higher penetrations of wind generation, penetration may be better measured as peak wind output over low demand plus export and storage. Variations on this approach may more accurately capture the likelihood of "excess" supply during periods of high wind output, and the ability of the system to economically absorb additional wind.

Electricity demand is variable but generally very predictable on larger grids; errors in demand forecasting are typically no more than 2% in the minutes-hours-day ahead timeframe. Depending on the demand profile and location, local weather conditions - particularly temperature - may be the primary driver of demand, and the sensitivity of demand to prediction errors may be well understood. Wind energy production can also be forecast, but there is considerably less experience predicting wind speeds, and the time frame of forecasts and sensitivity factors less well understood. At present, error rates for predicting wind production at important timeframes for grid operators (hours and day-ahead) are significantly higher than for demand predictions.

The maximum proportion of wind power allowable in a power system will thus depend on many factors, including the size of the system, the attainable geographical diversity of wind, the conventional plant mix (coal, gas, nuclear, hydroelectric) and seasonal load factors (heating in winter, air-conditioning in summer) and their statistical correlation with wind output. For most large systems the allowable penetration fraction (wind nameplate rating divided by system peak demand) is thus at least 15% without the need for energy storage. In addition, the interconnected electrical system may be much larger than the particular country or state (e.g. Denmark, California) being considered. A study published in October, 2006, by the Ontario Independent Electric System Operator (IESO) found that "there would be minimal system operation impacts for levels of wind capacity up to 5,000 megawatts (MW)," which corresponds to 17% of projected peak load (nameplate wind capacity over peak load); at the time of publication, Ontario had only 300 MW of installed wind capacity. While there are both practical and theoretical upper limits (as with any type of electric power generation), these upper limits are frequently many times higher than existing installed capacity.

A new press release about a Study of the Grid in Ireland (North and
South) indicates that it would be feasible to accommodate 42% renewables
in the electricity mix.

Wind power generation tends to be higher in the winter and at night (due to higher air density), so the appropriateness of wind power in high concentrations may crucially depend on the prevalence of air conditioning in a given jurisdiction. Wind power may be weakest in the hot summer months, and particularly during the day when air conditioning demand is highest. Conversely, systems where heat is electrical may be well-suited to higher penetration of wind power.

Geographic diversity

The variability of production from a single wind turbine can be high. Combining any additional number of turbines (for example, in a wind farm) results in lower statistical variation, as long as the correlation between the output of each turbine is imperfect, and the correlations are always imperfect due to the distance between each turbine. Similarly, geographically distant wind turbines or wind farms have lower correlations, reducing overall variability. Since wind power is dependent on weather systems, there is a limit to the benefit of this geographic diversity for any power system.

While wind power is variable, it is reliable in the sense that simultaneous failure of a large number of units (and the associated loss of generation capacity) is unlikely. It is highly improbable that a large number of wind turbines could fail simultaneously; concentrated transmission systems may be more likely points of failure. This should be contrasted with large single-source generators (nuclear, fossil, or other), which can go off-line in short periods of time; all power systems incorporate some reserve to compensate for such "single-source" losses of generating capacity.

Multiple wind farms spread over a wide geographic area and gridded together produce power more constantly and with less variability than smaller installations. Wind output can be predicted with some degree of confidence using weather forecasts, especially from large numbers of turbines/farms. The ability to predict wind output is expected to increase over time as data is collected, especially from newer facilities.

Compensating for variability

As noted, all sources of electrical power have some degree of unpredictability, and demand patterns (while relatively predictable) routinely drive large swings in the amount of electricity that suppliers feed into the grid. Wherever possible, grid operations procedures are designed to match supply with demand at high levels of reliability, and the tools to influence supply and demand are well-developed. The introduction of a power source in large amounts, however, that has a random element, relatively large differences between peak and trough output, and that may not be well-matched to demand cycles may require changes to existing procedures and additional investments.

Operational reserve

At times of high or increasing demand where wind output may simultaneously be falling, a number of solutions are either commonly used today or potentially feasible. Since all managed grids already have existing operational and "spinning" reserve, the amount of incremental reserve required for wind may be inconsequential. Contrary to perceptions, the addition of intermittent resources such as wind does not require 100% "back-up" or reserve plant; operating reserves and balancing requirements are calculated on a system-wide basis, and not dedicated to specific generating plant.

Because conventional powerplants can drop off the grid within a few seconds, for example due to equipment failures, in most systems the output of some coal or gas powerplants is intentionally part-loaded to follow demand and to replace rapidly lost generation. The ability to follow demand (by maintaining constant frequency) is termed "response." The ability to quickly replace lost generation, typically within timescales of 30 seconds to 30 minutes, is termed "spinning reserve." Nuclear power plants in contrast are not very flexible and are not intentionally part-loaded. A power plant that operates in a steady fashion, usually for many days continuously, is termed a "base load" plant. Generally thermal plants running as "peaking" plants will be less efficient than if they were running as base load. Hydroelectric facilities with storage capacity (such as the traditional dam configuration) may be operated as base load or peaking plants, and complement high levels of wind penetration.

In practice, as the power output from wind varies, part-loaded conventional plants, which must be there anyway to provide response (due to continuously changing demand) and reserve, adjust their output to compensate; they do this in response to small changes in the frequency (nominally 50 or 60 Hz) of the grid.

Demand reduction or increase

Energy Demand Management or Demand-Side Management refers to the use of communication and switching devices which can release deferrable loads quickly, or absorb additional energy to correct supply/demand imbalances. Incentives have been widely created in the American, British and French systems for the use of these systems, such as favorable rates or capital cost assistance, encouraging consumers with large loads to take them off line or to start diesels whenever there is a shortage of capacity, or conversely to increase load when there is a surplus, and clearly these same systems can be extended to enable power users to take advantage of renewable energy by adjusting their loads to coincide with resource availability. For example, pumping water to pressurize municipal water systems is an electricity intensive application that is already manipulated to coincide with times when electricity is cheaply available. Real-time variable electricity pricing can encourage all users to reduce usage when the renewable sources happen to be at low production.

See Load Control for further detail on existing load control - one US example cited can drop 2Gw in an emergency.

Instantaneous demand reduction. Most large systems also have a category of loads which instantly disconnect when there is a generation shortage, under some mutually beneficial contract. This can give instant load reductions (or increases). See National Grid (UK) reserve service

Storage and demand loading

At times of low or falling demand where wind output may be high or increasing, grid stability may require lowering the output of various generating sources or even increasing demand, possibly by using energy storage to time-shift output to times of higher demand. Such mechanisms can include:

Long-term storage of electrical energy involves substantial capital costs, space for storage facilities, and some portion of the stored power will be lost during conversion and transmission. The percentage retrievable from stored power is called the "efficiency of storage." The cost of compensating for the variability of wind has been studied extensively at low to medium penetrations, but would be expected to rise with higher penetration levels; the increase in costs with significantly higher penetration may be non-linear as the variability becomes more significant at higher levels, and particularly if storage needs to be purpose-built for wind. Pumped storage hydropower is the most prevalent existing technology used, and can substantially improve the economics of wind power.See also: Grid energy storage

In energy schemes with a high penetration of wind energy, secondary loads, such as desalination plants and electric boilers, may be encouraged because their output (water and heat) can be stored. The utilization of "burst electricity", where excess electricity is used on windy days for opportunistic purposes greatly improves the economic efficiency of wind turbine schemes. An ice storage device has been invented which allows cooling energy to be consumed during resource availability, and dispatched as air conditioning during peak hours. Various other potential applications are being considered, such as charging plug-in electric vehicles during periods of low demand and high production; at present, the scale at which such technologies are employed is relatively low.

In Colorado, a test facility financed with the cooperation of the National Renewable Energy Laboratory will produce hydrogen from wind power that will be used for electricity production during peak hours. This hydrogen could also be used in hydrogen vehicles. Production costs are estimated at $8 per kilogram (roughly the equivalent of one U.S. gallon of gasoline), or approximately "three times as expensive as using gasoline". This cost estimate appears to be based on retail gasoline prices of approximately $2.65 per gallon, and without consideration of pollution externality; no public cost breakdown is available. Although this cost may not be considered economically competitive at present, the costs may come down in future as the technology is proven. For a grid with "excess" wind power, any additional marginal revenue from producing hydrogen, for example, may still improve the economics of wind and hence allow for higher penetrations.

Complementary power sources and matching demand

Electricity produced from solar energy could be a counter balance to the fluctuating supplies generated from wind. In some locations, it tends to be windier at night and during cloudy or stormy weather, so there is likely to be more sunshine when there is less wind.

In some locations, electricity demand may have a high correlation with wind output, particularly in locations where cold temperatures drive electric consumption (as cold air is denser and carries more energy).

The allowable penetration may be further increased by increasing the amount of part-loaded generation available. Systems with existing high levels of hydroelectric generation may be able to incorporate substantial amounts of wind, although high hydro penetration may indicate that hydro is already a low-cost source of electricity; Norway, Quebec, and Manitoba all have high levels of existing hydroelectric generation (Quebec produces over 90% of its electricity from hydropower, and the local utility, Hydro-Québec, is the largest single hydropower producer in the world). The US Pacific Northwest has been identified as another region where wind energy is complemented well by existing hydropower, and there were "no fundamental technical barriers" to integrating up to 6000 MW of wind capacity. Storage capacity in hydropower facilities will be limited by size of reservoir, and environmental and other considerations.

Existing European hydroelectric power plants can store enough energy to supply one month's worth of European electricity consumption. Improvement of the international grid would allow using this in the relatively short term at low cost, as a matching variable complementary source to wind power. Excess wind power could even be used to pump water up into collection basins for later use. In practice, Denmark's system is well-integrated with the hydro-electric dominated Norwegian system, and Norwegian hydropower is used to balance fluctuations and shortfalls in Denmark; on occasion, Denmark exports electricity to Norway when generation is higher than demand (thereby increasing stored hydropower). Increased wind penetration may raise the value of existing peaking or storage facilities and particularly hydroelectric plants, as their ability to compensate for wind's variability will be under greater demand.

Export & import arrangements with neighboring systems

It is often feasible to export energy to neighboring grids at times of surplus, and import energy when needed. This practice is common in Western Europe and North America.

This linking of systems means that the relevant "penetration area" for wind systems may be considerably larger than the political entity or grid operator's area, making penetration for wind even less an issue. Denmark's 44% penetration, in the context of the German/Dutch/Scandinavian grids with which it has interconnections, is considerably lower as a proportion of the total system.

Since correlation between wind turbine outputs decreases with distance, the integration of grids may decrease the overall variability, as long as the wind turbines themselves are located at a considerable distance and not in the same weather system. For example, Germany's wind farms are predominantly located in the north of the country, reducing the value of the distance benefit due to the proximity to Denmark.

If neighboring grids both have significantly high levels of wind generation, and correlations between turbines are greater than zero, the likelihood that both will experience periods of high or low wind output at the same time may reduce the value of export/import arrangements, and also depend on the generation profile of other sources.

Transmission capacity for export may have to be substantially upgraded. Substantial transmission upgrades are already required within many countries, in some cases partly due to plans to significantly increase wind capacity.

Ultimately, the ability to export at will presupposes the presence of sufficient demand in the export market at prices sufficient to justify the investment in wind capacity and any additional transmission capacity required. At small and moderate penetration levels this may be a reasonable assumption, but at very high penetration levels will need to be demonstrated in practice.

Likely cost of grid reinforcement to deal with intermittency

In addition to interconnection with adjacent grid, grids will have to be upgraded internally, partly to bring power from remote renewable generators to load centres and partly to smooth out regional power variations. The costs of this are likely to be modest see 'Costs of reinforcing the National Grid to cope with e.g. Renewable Energy' National Grid (UK) where it is estimated that to double the capacity of the UK grid would add about 0.2p/kWh

In the USA it is estimated that to upgrade the transmission system to take in planned or potential renewables would cost at least $60 billion. Total annual US power consumption in 2006 was 4 thousand billion kilowatt hours. . Over an asset life of 40 years and low cost utility investment grade funding, the cost of $60 billion investment would be about 5% p.a. ie $3 billion p.a. Dividing by total power used gives an increased unit cost of around $3,000,000,000 x 100 / 4,000 x 1 exp9 = 0.075 cent / kWh.

Maximum penetration limits

There is no generally accepted maximum penetration of wind energy that would be feasible in any given grid. Rather, economic efficiency and cost considerations are more likely to dominate as critical factors; technical solutions may allow higher penetration levels to be considered in future, particularly if cost considerations are secondary.

High penetration scenarios may be feasible in certain circumstances:

Power generation for periods of little or no wind generation can be provided by retaining the existing power stations. The cost of using existing power stations for this purpose may be low since fuel costs dominate the operating costs. The actual cost of paying to keep a power station idle, but usable at short notice, may be estimated from published spark spreads and dark spreads. As existing traditional plant ages, the cost of replacing or refurbishing these facilities will become part of the cost of high-penetration wind if they are used only to provide operational reserve.

The aggregate maximum rate of change of generation from a close to 100% wind scenario may be lower than the existing rate of change of total generation due to unscheduled power station outages (for example, in the UK). The aggregate rate of change of output of such a scenario may also be smaller than the rate at which power stations can be warmed / started and ramped up, and successive short term weather forecasts (from 12 hours for an initial forecast to 5 minutes for final balancing) may be sufficient to schedule sufficient running plant, warming plant and spinning reserve plant.

Automatic load shedding of large industrial loads and its subsequent automatic reconnection is established technology and used in the UK and US, and known as Frequency Service contractors in the UK. Several GW are switched off and on each month in the UK in this way. Reserve Service contractors offer fast response gas turbines and even faster diesels in the UK, France and US to control grid stability.

In a close-to-100% wind scenario, surplus wind power can be allowed for by increasing the levels of the existing Reserve and Frequency Service schemes whereby a rise in system frequency would automatically connect loads, and disconnect them later, and by extending the scheme to domestic-sized loads. This means energy can either be stored by advancing deferrable domestic loads such as storage heaters, water heaters, fridge motors, or even hydrogen production. This would still result in lower revenue for wind generation and involve additional costs, but is in theory technically feasible. Under a high-penetration wind scenario, the amount of load shedding and reconnection would likely be several times higher, presenting challenges for system stability.

Alternatively or additionally, power can be exported to neighboring grids and re-imported later. HVDC cables are efficient with 3% loss per 1000 km and may be inexpensive in certain circumstances. For example an 8 GW link from UK to France would cost about £1 billion using high-voltage direct current cables. Under such scenarios, the amount of transmission capacity required may be many times higher than currently available.

One paper, which is a detailed study of a Europe wide renewable energy strategy, indicates that 100% penetration of renewables could be achieved at a modest cost - e.g. 3% extra total annual expenditure. This implies there is no limit for penetration.

A new press release about a Study of the Grid in Ireland (North and South) indicates that it would be feasible to accommodate 42% renewables in the electricity mix (at a price).

Intermittency and renewable energy

There are differing views about renewable energy and intermittency. The World Nuclear Association argues that the sun, wind, tides and waves cannot be controlled to provide directly either continuous base-load power, or peak-load power when it is needed. Proponents of renewable energy use argue that the issue of intermittency of renewables is over-stated, and that practical experience demonstrates this.

Views of critics of high penetration renewable energy use

The World Nuclear Association states that "Sun, wind, tides and waves cannot be controlled to provide directly either continuous base-load power, or peak-load power when it is needed. In practical terms they are therefore limited to some 10-20% of the capacity of an electricity grid, and cannot directly be applied as economic substitutes for coal or nuclear power, however important they may become in particular areas with favourable conditions." "The fundamental problem, especially for electricity supply, is their variable and diffuse nature. This means either that there must be reliable duplicate sources of electricity, or some means of electricity storage on a large scale. Apart from pumped-storage hydro systems, no such means exist at present and nor are any in sight." "Relatively few places have scope for pumped storage dams close to where the power is needed, and overall efficiency is low. Means of storing large amounts of electricity as such in giant batteries or by other means have not been developed."

On December 10, 2007 Patrick Moore, a co-founder and former leader of Greenpeace, wrote the following, "Greenpeace is deliberately misleading the public into thinking that wind and solar energy, both of which are inherently intermittent and unreliable, can replace baseload power that is continuous and reliable. Only three technologies can produce large amounts of baseload power: fossil fuels, hydroelectric plants and nuclear power. Given that we want to reduce fossil fuels and that potential hydroelectric sites are becoming scarce, nuclear power is the main option... Over the past 10 years, Germany and Denmark have poured billions of taxpayers' euros into wind and solar energy in the vain hope that this would allow them to shut down fossil fuel and nuclear plants. They have not succeeded because every solar panel and every wind turbine must be backed up by reliable power when the sun isn't shining and the wind isn't blowing." However, Mr. Moore's last involvement with Greenpeace was in Canada some 23 years ago, and he currently heads the Clean & Safe Energy Coalition, an astroturfing group funded by the US Nuclear Energy Institute.

Views of proponents of high penetration renewable energy use

Some renewable electricity sources have identical variability to coal-fired power stations, so they are base-load, and can be integrated into the electricity supply system without any additional back-up. Examples include:

Bio-energy, based on the combustion of crops and crop residues, or their gasification followed by combustion of the gas.

Furthermore, supporters argue that the total electricity generated from a large-scale array of dispersed wind farms, located in different wind regimes, cannot be accurately described as intermittent, because it does not start up or switch off instantaneously at irregular intervals. With a small amount of supplementary peak-load plant, which operates infrequently, large-scale distributed wind power can substitute for some base-load power and be equally reliable.

Hydropower can be intermittent and/or dispatchable, depending on the configuration of physical plant. Typical hydroelectric plants in the dam configuration may have substantial storage capacity, and be considered dispatchable. Run of the river hydroelectric generation will typically have limited or no storage capacity, and may be intermittent on a seasonal or annual basis (dependent on rainfall and other factors).

Amory Lovins suggests a few basic strategies to deal with these issues:

"The variability of sun, wind and so on, turns out to be a non-problem if you do several sensible things. One is to diversify your renewables by technology, so that weather conditions bad for one kind are good for another. Second, you diversify by site so they're not all subject to the same weather pattern at the same time because they're in the same place. Third, you use standard weather forecasting techniques to forecast wind, sun and rain, and of course hydro operators do this right now. Fourth, you integrate all your resources — supply side and demand side...

Several studies have demonstrated the technical feasibility of integrating intermittent power at levels substantially higher than is common in most countries (from 15-30% penetration), and at least three countries have more than 20% wind penetration. Relatively few changes to large grids are normally required and the associated system costs are moderate. International groups are studying much higher penetrations (30-75%, corresponding to up to 20% of national electricity consumption) and preliminary conclusions are that these levels are also technically feasible. In the UK, one summary of other studies indicated that if assuming that wind power contributed less than 20% of UK power consumption, then the intermittency would cause only moderate cost.

Methods to manage wind power integration range from those that are commonly used at present (e.g. demand management) to potential new technologies for grid energy storage. Improved forecasting can also contribute as the daily and seasonal variations in wind and solar sources are to some extent predictable.

Energy resilience

Diversity and dispersal also add system security. If one wind turbine fails, the lights won't flicker. If an entire windfarm gets knocked out by a storm, only 40,000 people will lose power. If a single Darlington reactor goes down, 400,000 homes, or key industries, could face instant blackouts. To hedge this extra risk, high premiums have to be paid for decades to ensure large blocks of standby generation

Power tower

Solar power can evaporate (thermal) or pump up (photovoltaic) water that can be stored behind a dam or tower to produce electricity later .